mpi3mr: Remove unused fields in struct mpi3mr_cmd

All of these fields are either unused, or just initialized. Remove
them. This saves about 1MB of memory for the cards that I have which can
do 8k transactions at once.

Sponsored by: Netflix
Reviewed by: mav, jhb
Differential Revision: https://reviews.freebsd.org/D42538

(cherry picked from commit a2b046620c54db977196128b3c53da2704b9fd20)